1. Field of the Invention
The present invention relates to a rotating speed control device of a rotary type electrostatic spray painting device.
2. Description of the Related Art
A rotary type electrostatic spray painting device which is equipped with a rotary shaft having a spray head and driven by an air turbine is known (for example, Japanese Unexamined Patent Publication No. 57-113860). Where such a rotary type electrostatic spray painting device is used in a painting process for a vehicle body, it is necessary to frequently change the colors of paint to be painted by such painting device. In addition, since an optimum rotating speed of the spray head exists for each color paint, it is necessary to change the rotating speed of the spray head every time the color of the paint to be painted is changed. To this end, in such a rotary type electrostatic spray painting device, the nozzle of the air turbine is connected to the pressurized air source via a plurality of air supply conduits, and a solenoid valve and an air jet forming a restricted opening are arranged in each air supply conduit. The flow areas of the air jets are different from each other. The rotating speed of the spray head is controlled in such a way that the amount of pressurized air is changed stepwise by selectively opening one of the solenoid valves in accordance with a change in color of the paint to be painted.
However, where the amount of pressurized air fed into the air turbine is controlled due to differences in the flow areas of the air jets, when the pressure of the pressurized air source fluctuates, or when the load acting on the shaft fluctuates due to a change in the viscosity of the paint, the rotating speed of the rotary shaft changes. Therefore, a problem occurs in that it is difficult to maintain the rotating speed of the spray head at a predetermined optimum speed.